1. Technical Field
The present invention relates to a broadband optical array multiplexer and, more particularly, to a reflective array structure that provides polarization compensation and yields an array of substantially reduced wafer area when compared with prior art multiplexers.
2. Description of the Prior Art
Optical wavelength division multiplexers, in particular "dense" multiplexers have become standard optical components that are capable of being implemented as planar lightguide circuits using planar lightwave circuit (PLC) technology. The basic dense wavelength multiplex (DWDM) router is disclosed in U. S. Pat. No. 4,904,042 issued to Corrado Dragone on Feb. 27, 1990. Often referred to as a "Dragone router" the disclosed N.times.N router is capable of launching an optical signal into any one of N inputs and, by controlling the wavelength, allowing the signal to appear at one of N outputs. The device is reciprocal in form and as such is also capable of performing as a demulitplexer. The capability to provide routing between any arbitrary input and output is achieved by forming the coupler to have opposing circular surfaces with each circular surface passing through the center of curvature of the other. FIG. 1 is an illustration of an exemplary prior art Dragone router.
The planar nature of the waveguide construction of routers such as that illustrated in FIG. 1 tends to produce a degree of polarization dependence in the group index of refraction of each waveguide. In any event, there is often observed a polarization-dependent offset of wavelength dependent characteristics. Also, the size of some Dragone routers becomes problematic as N increases, since there is a minimum bend radius that a given waveguide technology will support.
One approach to solving the polarization-dependent problem, proposed by H. Takahashi et al. in an article entitled "Polarization Insensitive Arrayed-Waveguide Grating Wavelength Multiplexer on Silicon" appearing in Optics Letters, Vol. 17, No. 7, April 1992, is to insert a birefringent quartz plate at the midpoint between successive routers so that the TE and TM polarizations are interchanged. FIG. 2 is a prior art illustration of this proposal. Although the effects of birefringence will be cancelled with this arrangement, the insertion of the quartz plate results in adding a loss of approximately 5 db to the signal path. Also, the capability of processing the silicon substrate to form a slot for the quartz plate is somewhat problematic.
A need remains, therefore, for an optical router architecture that is more compact than a conventional Dragone router and does not exhibit the polarization-dependent characteristics of such routers.